A variety of methods have been proposed for the remediation of soils containing organic contaminants. Many of the proposed methods involve evacuation and subsequent incineration of the soil with attendant difficulties of treatment or disposal of the off-gases. The total cost of such processes is high, often exceeding 500 U.S. dollars per ton of soil.
To avoid at least a portion of these problems, several types of in-situ processes have been proposed including vitrification of the soil by electrode heating, steam or hot air heating of the soil through an auger system or through stationary pipes, radio-frequency heating and electrical heating by means of a surface heater.
Brouns et al, U.S. Pat. No. 4,376,598, disclose a vitrification process in which the soil is heated to approximately 1500.degree. C. At or about this temperature the soil forms a glass-like mass which traps the contaminants within. This process, in reality, is a stabilization process rather than a decontamination process since the soil undergoing treatment has lost its physical and chemical identity. Both an auger system for injecting steam or hot air and a process of steam injection from stationary pipes have been practiced commercially. These methods are best applied to small areas of soil containing deep contamination such as localized spills at service stations. The methods are not as useful when applied to large areas of soil or to soil containing relatively shallow contamination. Bridges et al, U.S. Pat. No. 4,670,634, disclose an in-situ thermal process for soil decontamination where the thermal energy is supplied by radio-frequency heating. The process is particularly applicable to water-containing soils where the steam generated in the soil serves to strip organic contaminants from the soil. A somewhat related process is shown by copending U.S. patent application Ser. No. 427,418, filed Oct. 27, 1989, now U.S. Pat. No. 4,984,594, wherein the thermal energy is supplied by means of a relatively flat heater deployed at the surface of the soil.
Each of these in-situ methods is hampered by requirements for lengthy periods of heating and often multiple treatments are required. The methods that depend upon the presence of moisture in the soil are relatively inefficient in arid or semi-arid areas without the addition of large volumes of water. Moreover, each of the methods require continuous use of vapor collection systems to collect the contaminants desorbed from the soil and some means to treat or dispose of the material collected.
In the Bridges et al patent, general process mechanisms are discussed such as pyrolysis, oxidation and chemical reaction by which the chemical nature of soil contaminants is altered. However, the teachings are general in nature and no disclosure is made of specific methods for effecting low temperature degradation of soil contaminants. It would be of advantage to provide a simplified, relatively low temperature in situ process for soil decontamination with attendant simplified off-gas control.